a. Field of the Invention
The invention relates to the construction of resistor ratios, and more particularly to a circuit for use in integrated circuits in achieving a desired resistor ratio.
B. Prior Art
In the manufacture of integrated circuits, resistor ratios are frequently provided wherein a first resistor of a ratio pair is constructed during one processing step, while a second member of the pair is constructed during a second processing step. Because of variations in the processing steps necessary to accommodate other circuit elements being manufactured during the same processing steps, the errors in the resistance values differ from each other and do not cancel when the final ratio pair is formed. Moreover, construction of the desired resistor ratio is not repeatable because even the same process steps vary significantly from one operation to the next.
One approach to constructing desired resistor ratios is to minimize processed induced changes by optimizing the resistor geometry. Such an approach is described in U.S. Pat. No. 3,771,095 granted Nov. 6, 1973. This patent teaches the use of geometries which have different, rather than the same, widths. While this is an interesting approach, it is considered to be good manufacturing practice that resistors which must conform to a desired ratio must also be of similar length and width in integrated circuit construction.
U.S. Pat. No. 3,659,121, issued Apr. 25, 1972 teaches that widely varying resistor ratios impose a severe area penalty in integrated circuit construction and that the problem may be solved by providing a constant current source generating a very small current. This obviates the need for a large resistor of a ratio pair. This is of some interest where the resistors of the ratio pair do not form a voltage divider. However, where a voltage divider is contemplated, both members of the ratio pair are necessary.
It is the object of the present invention to devise a construction for a resistor ratio pair and to compensate for errors in the ratio pair. It is a further object of the invention to provide a circuit which maintains a predictable voltage across a load resistor of a resistor ratio pair.